Process for adherently depositing a metal carbide on a metal substrate



Feb. 13, 1968 .J. R. DARNELL. ET AL 3,368,914

PROCESS FOR ADHERENTLY DEPOSITING A METAL CARBIDE ON A METAL SUBSTRATE Filed Aug. 5, 1964 A L L w 22 55 52 INERTGAS WF 9 A WF 9 6 20 VARIABLE 6 ELECTRIC POWER 2 SUPPLY H2 OR OR INERT GAS INERT GAS J fi ia k ames [Mme BY flawa/a/af VZWMW WW fi w/zw/ 0% away United States Patent 3,368,914 PROCESS FOR ADHERENTLY DEPOSITING A METAL CARBIDE ON A METAL SUBSTRATE James R. Darnell and Donald A. Tarver, Richardson, Tex., assignors to Texas Instruments Incorporated, Dallas, Tern, a corporation of Delaware Filed Aug. 5, 1964, Ser. No. 387,725 Claims. (Cl. 1177 1) ABSTRACT OF THE DISCLOSURE A process involving the diffusion of a metal, such as tungsten, into a metal substrate, such as steel, followed by the depositing of a carbide of the diffused metal onto the substrate by chemical vapor deposition. After the diffusion of the metal into the substrate, a film of the diffused metal may be deposited on the substrate surface followed by the depositing of a carbide of the diffused metal on said film.

The present invention relates to metal carbide products and more particularly, but not by way of limitation, to a process for adherently bonding relatively thick metal carbide coatings to relatively thick metal substrates, and more specifically, to bonding tungsten carbide coatings to steel substrates.

In copending U.S. application Ser. No. 387,613, filed by Donald A. Tarver on even date herewith, entitled CVD Process for Producing Tungsten Carbide and Article of Manufacture, and assigned to the assignee of the present application, a process for producing tungsten carbide, molybdenum carbide, and other metal carbides is disclosed and claimed. In general, the disclosed process comprises reacting a metal halide with carbon monoxide, preferably in the presence of hydrogen, to produce a coherent metal carbide. In particular, the disclosed process involves the reaction of tungsten hexafiuoride and carbon monoxide to produce tungsten carbide.

Tungsten carbide is particularly. useful for tipping machine cutting tools due to its extreme hardness but is too brittle to be useful for this purpose unless backed by the tensile strength of a metal substrate such as steel. The adherent bonding of tungsten carbide to a metal substrate, and in particular to steel, presents a considerable problem since the coefficient of linear thermal expansion of carbon steel is almost two and one-half times as great as that of tungsten carbide. Therefore when the steel substrate is cooled after the tungsten carbide has been deposited at high temperature, the tungsten carbide tends to spall off. As described in the abovereferenced co-pending application, the tungsten carbide can be made readily adherent to a steel substrate provided either the tungsten carbide coating or the steel substrate is relatively thin because the thin member will deform sufficiently to compensate for the difference in thermal contraction. It is also disclosed in the above-referenced co-pending application that the tungsten carbide can be better bonded to the steel substrate by a thin film of tungsten metal laid down on the substrate by a chemical vapor deposition process, such as by the reduction of tungsten hexafluoride in the presence of hydrogen.

The present invention is concerned with a process for adherently bonding a coat of tungsten carbide, or other metal carbide of substantial thickness, to a steel, or other metal substrate, also of substantial thickness, so that the resulting article will have a very hard surface, yet will have considerable tensile strength. In accordance with the present invention, a metal carbide coating is adherently applied to the surface of a metal substrate by first diffusing a second metal into the surface of the substrate so as to relax the thermal expansion coefficient of the surface zone of the substrate. Then the metal carbide is deposited on the diffused surface of the substrate by a chemical vapor deposition process. In general, it is preferable to diffuse the metal forming the carbide into the substrate. For example, in the situation where tungsten carbide is to be adherently bonded to a steel substrate, tungsten is preferably diffused into the steel substrate prior to the deposition of the tungsten carbide.

The tenacity of the bond between the metal carbide and the metal substrate may be substantially enhanced by also depositing a thin film of a second metal over the surface of the diffused substrate. Again it is preferred that the metal film be the metal forming the carbide. Thus tungsten is diffused into the surface of the steel substrate, then a thin film of tungsten is deposited on the diffused surface. The tungsten carbide coat is then deposited on the tungsten film. It is also preferable to partially diffuse the tungsten metal film into the already diffused surface of the substrate.

The process of this invention is generally useful for bonding tungsten carbide, molybdenum carbide, and other carbides to substantially any metal substrate, and in particular for bonding tungsten carbide and molybdenum carbide to iron and nickel-base alloys. In general, the bonding of a thick coat of tungsten carbide to a thick steel substrate represents the most difficult task because the variance between the coefficients of thermal expansions of these two materials is greater than between any other metal carbide and metal which might be of interest.

In accordance with a more specific aspect of the invention, tungsten is diffused into the surface of a steel substrate either by a pack diffusion process or by partially diffusing a tungsten film applied to the substrate by chemical vapor deposition. If the pack diffusion process is not used, a film of tungsten is preferably deposited on the substrate surface by a chemical vapor deposition process and then partially diffused by raising the substrate to a temperature in excess of the deposition temperature for a considerable period of time. Then finally the tungsten carbide is deposited on the tungsten film over the steel substrate by the reaction of tungsten hexafluoride and carbon monoxide in the presence of hydrogen.

Therefore, an important object of the present invention is to provide an improved process for adherently bonding tungsten carbide to a steel substrate, particularly when neither the steel substrate nor the tungsten carbide coating is sufficiently thin to elastically deform to accommodate the differences in thermal expansion and contraction of the two mated materials.

Still another important object of this invention is to provide a steel substrate having an adherent coat of coherent tungsten carbide over the surface thereof.

A further object of this invention is to provide a process for adherently depositing a metal carbide upon a metal substrate, particularly where both are so thick as not to elastically deform to compensate for a mismatch in thermal expansivity.

Many additional objects and advantages of the present invention will be evident to those skilled in the art from the following detailed description and drawing, wherein:

FIGURE 1 is a schematic diagram of a system which may be used to carry out the process of the present invention;

FIGURE 2 is a sectional view, somewhat schematic, of a portion of an article of manufacture constructed in accordance with the present invention; and

FIGURE 3 is a sectional view, somewhat schematic, of a portion of another article of manufacture constructed in accordance with the present invention.

Referring now to FIGURE 1, a system for carrying out the process of the present invention is indicated generally by the reference numeral 10. The system is comprised of a controlled atmosphere reaction chamber 12 which may be operated at atmospheric pressure in which a sub strate 14 to be coated may be supported. The substrate 14 may be heated by any suitable means, such as by resistive heating from the variable electric power supply 16. One conduit system for carrying out one specific process of the present invention is indicated generally by the reference numeral 18 and is comprised of valved conduits 20, 22 and 24 which are connected to a common inlet 26 to the chamber 12. The valved conduits 20, 22 and 24 are connected to a source of hydrogen or an inert gas, a source of tungsten hexafluoride gas and a source of carbon monoxide gas, respectively. A conduit system for carrying out another specific process in accordance with the present invention is indicated generally by the reference numeral 28 and is comprised of valved conduits 30 and 32 which are connected to sources of hydrogen or inert carrier gas and tungsten hexafluoride gas, respectively. A third valved conduit 34 is connected to a source of hydrogen or inert carrier gas and extends to the bottom of a container 36 containing particulate tungsten hexacarbonyl. The tungsten hexacarbonyl is heated by a suitable means (not illustrated) to establish the desired vapor pressure such that hydrogen passing through the container 36 will entrain the tungsten hexacarbonyl and be passed through the valved conduit 38 and mixed with the gases from the valved conduits 30 and 32 prior to introduction through the inlet 40 to the reaction chamber 12. A valved outlet conduit 42 is provided for Withdrawing gases from the chamber 12 in order to establish a flow of reactants past the substrate 14.

In carrying out one specific embodiment of the invention, a steel substrate 14 is properly cleaned, placed in the reaction chamber 12, and connected to the source of electrical power 16 so as to be resistively heated. It is to be understood, however, that the substrate 14 may be heated in any suitable manner, such as for example, by inductive or radiant heating techniques. Next the valved conduit is opened so as to admit hydrogen to the chamber 12 and the valved conduit 42 opened so that the chamber is purged of foreign gases. Next the substrate 14 is heated to a temperature greater than 400 C., and preferably between about 600 C. and about 1,000 C., such as for example, 800 C. The valved conduit 22 is then opened so as to mix tungsten hexafluoride with the hydrogen stream from the valved conduit 20. A flow of a mixture of hydrogen and tungsten hexafiuoride is then established past the heated substrate 14 so as to deposit a film of tungsten on the surface of the substrate by the hydrogen reduction of the tungsten hexaffuoride to form elemental tungsten. The flow of tungsten hexafluoride is then stopped, and the substrate 14 raised to a temperature from about 1,000 C. to about 1,200 C. for a significant period of time, preferably for several hours, so that the tungsten metal will partially diffuse into the surface of the steel substrate.

Next the substrate 14 is cooled to a temperature at which a coat of coherent tungsten carbide may be deposited as described in detail in the above-referenced application. This entails admitting a mixture of hydrogen, tungsten hexafiuoride and carbon monoxide through the valved conduits 20, 22 and 24, respectively. The mixture of gases is introduced to the chamber 12 and passed across the substrate 14 and withdrawn through the outlet conduit 42 at the desired flow rate. The temperature of the substrate should be greater than about 400 C. and is preferably in the range from about 600 C. to about 1,000 C., but it is believed that the proces may go as high as the properties of the substrate will permit. However, the higher the temperature the faster the deposition rate and the greater the tendency to produce a rough coating having relatively long spikes protruding therefrom. In the alternative, the conduit system 28 may be used to deposit the tungsten carbide in which case the valved conduits 30, 32, 34 and 38 are opened so as to admit a reactant mixture of hydrogen, tungsten hexafluoride, and the vapors of tungsten hexacarbonyl. The parameters of temperature and flow are substantially the same as the corresponding parameters when using carbon monoxide in the system 18. The resulting coherent tungsten carbide will be adherently bonded to the surface of the steel substrate into which tungsten has previously been diffused.

An alternative process for diffusing tungsten or other metal into the surface of the steel substrate may be employed without departing from the broad aspects of the present invention. Specifically, tungsten may be diffused into the steel substrate 14 prior to placement of the substrate in the reaction chamber 12 by a pack diffusion process. In this process particulate tungsten is mixed with a halide and hydrogen compound, such as for example, an ammonium halide, diluted with an inert ingredient such as alumina. The substrate is packed in this mixture in a receptacle and heated to an elevated temperature of from about 1,000 C. to about 1,200 C. for several hours until the desired degree of diffusion is obtained.

In general, the degree of diffusion desired by either diffusion process is from zero at the inner face of the diffusion front a few mils deep in the substrate to a concentration as high as 50% at the surface of the substrate. The result is a steel substrate with a tungsten-rich case. The tungsten is believed to displace the molecules of iron and assume the positions of the iron molecules in the steel crystal structure. After the substrate 14 has been prepared by the diffusion of the tungsten, it is then placed in the reaction chamber 12 and a coat of tungsten carbide deposited over the diffused surface using either of the processes described briefly above and in greater detail in the above-referenced application, that is by the reaction of tungsten hexafluoride and carbon monoxide in the presence of hydrogen.

The tenacity with which the tungsten carbide is adherently bonded to a steel substrate may be materially increased by using the following alternative process. First tungsten is diffused into the substrate 14 using either the pack diffusion process or the chemical vapor deposition process, or any other suitable diffusion technique so as to produce a tungsten-rich surface region around the steel substrate as previously described. Then a thin film of tungsten is deposited on the diffused surface of the steel substrate by chemical vapor deposition or other technique. Finally the coat of tungsten carbide is deposited on the tungsten film by the reaction of tungsten hexafluoride and carbon monoxide in the presence of hydrogen.

In this case the entire process may be carried out Within the reaction chamber without disturbing the substrate 14. The substrate 14 is placed in the reaction chamber 12, the chamber purged with hydrogen, the substrate 14 raised to a temperature in the range from about 600 C. to about 800 C. and a mixture of hydrogen and tungsten hexafiuoride passed through the chamber to deposit a film of tungsten on the surface of the substrate. The substrate is then heated to a temperature from about 1,000 C. to about 1,200 C. for a substantial period of time, usually several hours to partially diffuse the tungsten into the surface of the steel substrate. The substrate is then cooled to deposition temperatures greater than 400 C. and preferably between about 600 C. and about 1,000 C. and a mixture of hydrogen, tungsten hexafluoride and carbon monoxide passed through the reaction chamber until a coating of tungsten carbide of the desired thickness is deposited on the surface of the tungsten film on the surface of the steel substrate 14.

Although the specific processes of the present inven tion have all been described with regard to a steel substrate, a tungsten metal diffusion, a tungsten metal intcrlayer and tungsten carbide final coating, it will be appreciated that in its broader aspects, the process also applies to other metal substrates, to other metal diffusion materials, to other metal films on the substrate, and other metal carbide coatings. In general, adherently bonding relatively thick tungsten carbide to the surface of a steel substrate represents the most difficult problem to be encountered due to the considerable difference between the coefficients of thermal expansions of the two materials. Tungsten carbide on steel also has the most important immediate commercial implications, and for this reason, a major portion of our efforts has been directed toward achieving a tungsten carbide-to-steel bond. The adherence of other metal carbides to other metal substrates, or the adherence of tungsten carbides to other metal substrates, or the adherence of other metal carbides to steel substrates all present less difficult tasks and accordingly considerable success may be expected in these areas by using the abovedescribed processes. The process has been successfully used to produce a molybdenum carbide coat adherently bonded to a steel substrate. The process is further particularly applicable to bonding tungsten carbide coatings to nickel-base alloys. In general, the term steel as used herein includes almost all iron-base alloys. The metal used for the intermediate metal film and diffused regions may be tungsten, molybdenum, chromium, niobium and tantalum, as well as most other refractory metals which have other physical properties compatible with the particular use intended for the resulting article and with the temperatures involved in the process.

The articles of manufacture resulting from the above process constitute an important aspect of the present invention. One such article is indicated generally by the reference numeral 50 in FIGURE 2 and is comprised of a metal substrate 140 having a second metal diffused into the surface region 52 of the substrate to relax the coefficient of thermal expansion of the substrate metal. A coat of coherent metal carbide 54 is adherently bonded to the diffused surface of the substrate. In particular, the article 50 is comprised of a steel substrate 14a, a tungstendiff used region 52 and a substantially coherent tungsten carbide coating 54 having no significant metal phase and a hardness which exceeds commercially available tungsten carbide.

Another article constructed in accordance with the pressent invention is indicated generally by the reference numeral 60 in FIGURE 3. The article 60 is comprised of a metal substrate 14b having a diffused surface region 62 rich in a second metal as a result of the diffusion of the metal into the surface region. A thin metallic layer 64 is adherently bonded to the diffused surface of the substrate 14b, and a coat of coherent, substantially pure metal carbide 66 is adherently bonded to the metal layer 64. In particular, the substrate 14b is preferably steel or iron base alloy, the diffused metal in the region 62 is preferably tungsten, the metal layer 64 is preferably tungsten, and the metal carbide coating 66 is preferably tungsten carbide. However, the substrates 14a and 1412 may be nickel or nickel-base alloys or substantially any other metal having the thermal and strength characteristics which might be of interest for a particular application of the metal carbide coating. In particular, the metal in the diffused regions 52 and 62 and the metal forming the layer 64 may be any combination of tungsten, molybdenum, chromium, niobium or tantalum.

Having thus described several preferred embodiments of the present invention, it will be evident to those skilled in the art that a highly useful process has been described for producing a novel and useful article having a coat of metal carbide adherently bonded to the surface of a metal body. The article has the combined properties of the two materials, namely the hardness of the metal carbide and the tensile strength of the metal body. The uses to which the articles may be placed are widely known and include, for example, machine tools and drill-proof armor for safes and the like.

Although several preferred embodiments of the invention have been described in detail, it is to be understood that various changes, substitutions and alterations can be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

We claim:

1. A process for adherently bonding a coat of a carbide of a metal selected from a first group of metals consisting of tungsten, molybdenum, chromium, niobium, or tantalum to the surface of a heated substrate selected from a second group of metals consisting of steel, nickel, or a nickel-base alloy, comprising the steps of:

diffusing said selected metal into the surface region of said selected substrate, and

depositing a coat of the carbide of said selected metal on the surface of said selected substrate by reacting a halide of said selected metal with carbon monoxide in the presence of hydrogen at the surface of said selected substrate.

2. The process as defined in claim 1 wherein said selected metal is diffused into the surface region of said selected substrate by a pack diffusion process.

3. The process as defined in claim 2 wherein said selected metal is tungsten.

4. The process as defined in claim 1 wherein said selected metal is diffused into said selected substrate by depositing a thin film of said selected metal on the surface of said selected substrate and heating said selected substrate for a period of time to cause said selected metal to partially diffuse into the surface region of said selected substrate.

5. The process as defined in claim 4 wherein said selected metal is tungsten.

6. The process as defined in claim 4 wherein the film of said selected metal is deposited by the hydrogen reduction of a halide of said selected metal.

7. The process as defined in claim 6 wherein said selected metal is tungsten and a halide of said selected metal is tungsten hexafiuoride.

8. The process as defined in claim 1 wherein said selected metal is tungsten and a halide of said selected metal is tungsten hexafluoride.

9. The process as defined in claim 1 wherein said selected metal is molybdenum and said selected substrate is steel.

10. A process for adherently bonding a coat of a carbide of a metal selected from a first group of metals consisting of tungsten, molybdenum, chromium, niobium, or tantalum to the surface of a heated substrate selected from a second group of metals consisting of steel, nickel, or a nickel-base alloy, comprising the steps of:

diffusing said selected metal into the surface region of said selected substrate, depositing a film of said selected metal on the diffused surface of said selected substrate, and

depositing a coat of the carbide of said selected metal on the film of said selected metal by reacting a halide of said selected metal with carbon monoxide in the presence of hydrogen at the surface of the film of said selected metal.

11. The process as defined in claim 10 wherein said selected metal is tungsten and a halide of said selected metal is tungsten hexafluoride.

12. The process as defined in claim 10 wherein said selected metal is molybdenum and said selected substrate is steel.

13. The process as defined in claim 10 wherein said selected metal is diffused into said selected substrate by a pack diffusion process.

14. The process as defined in claim 13 wherein said selected metal is tungsten.

15. The process as defined in claim 10 wherein said selected metal is diffused into said selected substrate by depositing a thin film of said selected metal on the sur- 7 face of said selected substrate and heating said selected substrate to an elevated temperature for a period of time.

16. The process as defined in claim 15 wherein said selected metal is tungsten.

17. The process as defined in claim 15 wherein the film of said selected metal is deposited by the pyrolysis of a halide of said selected metal in the presence of hydrogen.

18. The process as defined in claim 17 wherein said selected metal is tungsten and a halide of said selected metal is tungsten hexafiuoride.

19. The process as defined in claim 10 wherein the film of said selected metal is deposited on the ditfused surface of said selected substrate by the hydrogen reduction of a halide of said selected metal.

20. The process as defined in claim 19 wherein said selected metal is tungsten and a halide of said selected metal is tungsten hexafiuoride.

8 References Cited UNITED STATES PATENTS FOREIGN PATENTS 1938 Great Britain.

OTHER REFERENCES Powell et al., Vapoe Plating, John Wiley and Sons, Inc.,

5 New York, 1955, pp. 5258 and 71.

ALFRED L. LEAVITT, Primary Examiner.

C. K. WEIFFENBACH, W. H. LOUIE, 111.,

Assistant Examiners. 

